1. Field of the Invention
The present invention relates generally to a method of cutting threads in work by using a numerically controlled machine tool, and more particularly to a method of shortening the incomplete thread portion in threading pullout.
2. Description of the Related Art
A known method of cutting threads in work is illustrated in FIGS. 1-4. FIG. 1 shows a functional configuration diagram of a numerical control apparatus 1, such as a numerically controlled lathe (“NC lathe” hereafter) used for cutting threads in work. This numerical control apparatus 1 includes a program analyzer 2 and a thread cutting controller 3. The program analyzer 2 is responsible for analyzing a machining program and for outputting commands regarding the rotational speed of the main spindle to the spindle speed controller 4 so as to control the operation of the main spindle drive motor 5.
As the program analyzer 2 reads a thread-cutting command from the machining program, the thread cutting controller 3 controls the X-axis position controller 9 and the Z-axis position controller 10 based on the thread-cutting information contained in the thread-cutting command, such as the pitch and the cut-in amount, as well as the information regarding the rotational position of the main spindle received from the pulse generator (PG) 8 included with the main spindle drive motor 5.
The X-axis position controller 9 generates and sends a position command along the X-axis to the X-axis acceleration/deceleration controller 11 so as to cause the tool to cut the work to the commanded cut-in amount. The Z-axis position controller 10 generates position commands regarding the Z-axis as command values that vary with time in synchronization with the rotation of the main spindle and sends these commands to the Z-axis acceleration/deceleration controller 12 so that the tool may cut the work at the feed speed to provide the commanded pitch.
The X-axis and Z-axis acceleration/deceleration controllers 11 and 12 in turn use acceleration/deceleration constants read out from the acceleration/deceleration constant storage 13 to control the acceleration/deceleration of the X-axis drive motor 6 and the Z-axis drive motor 7. The acceleration/deceleration constants, which comprise information about the allowable acceleration in the direction of the Z-axis (the acceleration/deceleration capabilities of the machine tool in the direction of the Z-axis is lower than those in the direction of the X-axis), are pre-stored in the acceleration/deceleration constant storage 13 so as to provide biaxial synchronous feed. During the pullout in thread cutting, the allowable Z-axis acceleration is used for acceleration/deceleration time constant control in the directions of the X-axis and Z-axis (acceleration/deceleration by the method of moving averages).
FIGS. 2 and 3 show a method of cutting threads in work under the control of the thread cutting controller 3. Generally, a typical thread cutting cycle includes the following four steps a-d. In step a, the tool T is transferred in the direction of the X-axis perpendicularly to the axis A of the main spindle to position the tool T at the point where thread cutting is to be started (S1 in FIG. 3). In step b, in synchronization with the rotation of the main spindle (not shown), the tool T is transferred at a predetermined feed speed in the direction of the Z-axis in parallel to the axis A of the main spindle to cut the work W held on the main spindle with the tool T (S2).
In step c, a “pullout” operation is performed; i.e., the tool T is retracted or moved away from the work W in the direction of the X-axis (S3). In step d, the tool T is withdrawn in the direction of the Z-axis to bring the tool back to the reference point of the thread cutting cycle (S4). If tapered threads are cut on the work W, biaxial synchronous feed in the direction of the X-axis and Z-axis is performed in step b.
FIG. 4 shows the changes in the speed of the tool_in the directions of the X-axis and Z-axis according to the conventional method of thread cutting. The solid lines represent the speed commands of the numerical control apparatus 1, whereas the broken lines represent the speeds of the tool interpolated according to acceleration/deceleration time constant control. Of the acceleration/deceleration capabilities of the machine tool in the direction of the X-axis and Z-axis, generally, the machine tool has higher acceleration/deceleration capabilities in the direction of the X-axis, along which a less mass (the tool post) needs to be driven than along the Z-axis, along which a greater mass (the tool post and the saddle) needs to be driven. However, the acceleration/deceleration times in the directions of the X-axis and Z-axis are set to the same value Ta.
In step c, the tool T is accelerated in the direction of the X-axis for a pullout operation while the tool T is decelerated in the direction of the X-axis to stop the tool feed. Accordingly, until the tool T is completely disengaged from the work W, the feed in the direction of the Z-axis becomes inconsistent with the regular screw thread pitch, resulting in an incomplete thread portion at the end of the effective thread portion.
In Published Unexamined Patent Application No. 2007-319971, a method is proposed comprising the steps of setting an idle running region outside the effective thread portion and allowing the tool to overrun to the idle running region so as to perform a pullout operation without the tool in contact with the work (see paragraph [0023]).